1. Technical Field
The present invention relates generally to semiconductor fabrication, and more particularly, to an immersion lithography apparatus and method, and a lithographic optical column structure for conducting immersion lithography with at least the projection optics of the optical system and the wafer under the same pressure.
2. Related Art
The ongoing pursuit of fabricating smaller semiconductor devices has currently caused the semiconductor fabrication industry to pursue advancements relative to immersion photolithography rather than dry photolithography. For example, the current focus in the industry has shifted from dry 157 nanometer (nm) wavelength lithography technology to immersion (wet) 193 nm wavelength lithography technology, with the objective to achieve immersion photolithography at 157 nm. Immersion photolithography approaches include filling a space between the last projection lens and the wafer in a lithography tool with a fluid having a higher refractive index (n) than the conventional fluid, i.e., air. As the refractive index increases, so does the numerical aperture (NA) of the optical lithography tool. The increased numerical aperture results in increased resolution of the pattern transfer process, and thus the potential for smaller devices.
In one approach, the fluid used is a liquid such as water, which provides a higher refractive index than air (e.g., air n=1, water n=1.44). Unfortunately, the extension of immersion lithography to 157 nm wavelengths is complicated by the lack of transparent liquid media that can be used as a liquid immersion material, e.g., water absorbs 157 nm light. Other challenges to this approach include prevention of bubbles in the liquid during exposure, inadequate wetting, wafer contamination and complexity. Another approach that addresses, inter alia, the transparency issue is use of a supercritical fluid, such as disclosed in U.S. Pat. No. 5,900,354 to Batchelder. One fluid disclosed in that reference is xenon (Xe), which is transparent at 157 nm and has a refractive index of 1.38, which is suitable for the immersion application and forms a supercritical state at room temperature. This approach is promising because it provides adequate optical transparency at 157 nm, and also eliminates the bubble formation problem with liquid immersion systems. A challenge facing widespread implementation of supercritical fluid immersion systems, however, is that they require high pressure (e.g., >60 atmospheres) to obtain the supercritical state, which distorts lithographic optical elements. For example, the above-described device requires an optical element 26 (FIG. 2 thereof) for transitioning between the high pressure supercritical fluid and ambient air.
In view of the foregoing, there is a need in the art for an immersion lithography apparatus and method that avoid the problems of the related art.